P wave attenuation structure in the fault area of the 1995 Kobe earthquake

The three-dimensional (3-D) P wave attenuation structure in the Kobe epicentral area is determined using aftershock records from the 1995 Kobe (Hyogo-ken Nanbu) earthquake. Waveform data from both permanent seismic networks and portable stations, set up following the Kobe mainshock, were used. The observed P wave amplitude spectra were inverted for the spectral plateau value, the source corner frequency, and a frequency-independent t* operator assuming an omega (2) -type source model. The calculated t* operators were subsequently used for the computation of the 3-D absorption structure using a 3-D velocity model. Only high-quality amplitude spectra were used for the estimation of reliable attenuation parameters, resulting in a final data set of similar to 4100 t* operators. Two regions of high absorption were found in the upper part of the crust. One is below the northern part of Awaji Island and reaches down to a depth of 5 km. The other region, northeast of Kobe city, is along the Arima-Takatsuki tectonic line and extends along the Rokko, Kashiodani, and Yamada Faults. It extends northward to the Hokusetsu-Sanchi area. In both regions a high density of fault lineations is mapped, and high Poisson ratios are known. This suggests that partially saturated cracks cause the observed high attenuation.